Section 8.7 — Leaching Beds

(1) This Section applies to the construction of leaching beds.

(1) The design and installation of a shallow buried trench, Type A dispersal bed or Type B dispersal bed shall be carried out by a person competent in this field of work.

(1) A leaching bed shall not be located (a) in an area that has an average slope that exceeds one unit vertically to four units horizontally, (b) in soil or leaching bed fill having a percolation time of, (i) less than 1 min, or greater than 125 min if constructed as a shallow buried trench, or (ii) less than 1min, or greater than 50 min for all other leaching beds, or (c) in or on an area that is subject to flooding that may be expected to cause damage to the leaching bed or impair the operation of the leaching bed. (2) A leaching bed shall not be covered with any material having a hydraulic conductivity less than 0.01 m/day. (3) The surface of the leaching bed shall be shaped to shed water and together with the side slopes of any raised portion, shall be protected against erosion in such a manner as to not inhibit the evaporation and transpiration of waters from the soil or leaching bed fill, and to not cause plugging of the distribution pipe. (4) No part of a leaching bed shall be sloped steeper than 1 unit vertically to 4 units horizontally. (5) A leaching bed shall be designed to be protected from compaction or any stress or pressure that may result in, (a) the impairment or destruction of any pipe in the leaching bed, or (b) the smearing of the soil or leaching bed fill.

(1) Sentence (2) applies to the design and construction of a leaching bed with distribution pipes used within the leaching bed. (2) The header line and distribution pipes within a leaching bed shall be designed and constructed so that they can be detected by (a) magnetic means, (b) means of a 14 gauge TW solid copper light coloured plastic coated tracer wire, or (c) other means of subsurface detection.

(1) Leaching chambers are permitted for use in conjunction with an absorption trench, shallow buried trench, filter bed or Type A dispersal bed. (2) Leaching chambers shall comply with the dimension requirements for either a Type I or Type II leaching chamber listed in Table 8.7.2.3. (3) Leaching chambers shall conform to the requirements of IAPMO PS 63, “Plastic Leaching Chambers”. (4) The header line and leaching chambers within a leaching bed shall be designed and constructed so that they can be detected by (a) magnetic means, (b) means of a 14 gauge TW solid copper light coloured plastic coated tracer wire, (c) means of a 12 gauge copper clad steel light coloured plastic coated tracer wire, or (d) other means of subsurface detection. Table 8.7.2.3. Leaching Chamber Dimensions Forming Part of Sentence 8.7.2.3.(2) Type of Leaching Chamber Width, mm Height, mm Type I 380 to 410 280 to 305 Type II 555 to 575 300 to 320

(1) The total length of distribution pipe shall (a) not be less than 30 m when constructed as a shallow buried trench, or (b) not be less than 40 m for any other absorption trench. (2) Except as provided in Sentences (1), (3), and (4) every leaching bed constructed by means of absorption trenches shall have a total length of distribution pipe not less than the value determined by the formula, L = QT/200 where, L = total length of distribution pipe in metres, Q = the total daily design sanitary sewage flow in litres, and T = the design percolation time. (3) Except as provided in Sentence (1), where a leaching bed receives effluent from a Level II, Level III or Level IV treatment unit as described in Table 8.6.2.2., the leaching bed may have a total length of distribution pipe not less than the value determined by the formula, L = QT/300 where, L = total length of distribution pipe in metres, Q = the total daily design sanitary sewage flow in litres, and T = the design percolation time. (4) Except as provided in Sentence (1), where the leaching bed is constructed as a shallow buried trench, the total length of the distribution pipe shall not be less than the value determined by Table 8.7.3.1. Table 8.7.3.1. Length of Distribution Pipe in Shallow Buried Trench Forming Part of Sentence 8.7.3.1.(4) Percolation Time, (T) of Soil, min Length of Distribution Pipe, m 1 < T ≤ 20 Q/75 20 < T ≤ 50 Q/50 50 < T < 125 Q/30 where, Q = the total daily design sanitary sewage flow in litres, and T = the design percolation time.

(1) The total length of leaching chamber shall (a) not be less than 30 m when constructed as a shallow buried trench, or (b) not be less than 40 m for any other absorption trench. (2) Except as provided in Sentences (1) and (3), the total length of leaching chamber shall not be less than the value determined by the formula, L = QT/200, for a Type I leaching chamber, or L = QT/300, for a Type II leaching chamber, where, L = total length of leaching chamber in metres, Q = the total daily design sanitary sewage flow in litres, and T = the design percolation time. (3) When a treatment unit described in Article 8.6.2.2. is used in conjunction with a leaching chamber, the total length of leaching chamber shall not be less than the value determined by the formula, L = QT/300 where, L = total length of leaching chamber in metres, Q = the total daily design sanitary sewage flow in litres, and T = the design percolation time. (4) Except as provided in Sentence (1), where the leaching bed is constructed as a shallow buried trench, the total length of leaching chamber shall not be less than the value determined by Table 8.7.3.2. Table 8.7.3.2. Length of Leaching Chamber in Shallow Buried Trench Forming Part of Sentence 8.7.3.2.(4) Percolation Time, (T) of Soil, min Length of Leaching Chamber, m 1 < T ≤ 20 Q/75 20 < T ≤ 50 Q/50 50 < T < 125 Q/30 where, Q = the total daily design sanitary sewage flow in litres, and T = the design percolation time.

(1) Except as provided in Sentence (2), absorption trenches shall be (a) approximately the same length and not more than 30 m in length, (b) not less than 500 mm and not more than 1 000 mm in width, (c) not less than 600 mm and not more than 900 mm in depth, (d) centred not less than, (i) 1 600 mm apart where used in conjunction with (A) a distribution pipe, (B) a Type I leaching chamber, or (C) a Type II leaching chamber constructed in leaching bed fill, or (ii) 2 400 mm apart where used in conjunction with a Type II leaching chamber, other than a Type II leaching chamber constructed in leaching bed fill, (e) located so that the bottom of the absorption trench is not less than 900 mm above the high ground water table, rock or soil with a percolation time of more than 50 min, and (f) backfilled, after the installation of the distribution pipe or leaching chamber with leaching bed fill, so as to ensure that after the leaching bed fill settles, the surface of the leaching bed will not form any depressions. (2) Absorption trenches constructed as a shallow buried trench shall be (a) approximately the same length and not more than 30 m in length, (b) not less than 300 mm and not more than 600 mm in width, (c) not less than 300 mm and not more than 600 mm in depth, (d) centred not less than 2 000 mm apart, (e) not less than 900 mm at all points on the bottom of the absorption trench above the high ground water table or rock, and (f) backfilled, after the installation of the distribution pipe with leaching bed fill, so as to ensure that after the leaching bed fill settles, the surface of the leaching bed will not form any depressions.

(1) Except for a shallow buried trench, the distribution pipe used in the construction of a leaching bed shall be (a) not less than 3 in. trade size for gravity flow systems, (b) installed with a uniform downward slope from the inlet with a drop of not less than 30 mm and not more than 50 mm for each 10 m of distribution pipe for gravity flow systems, and (c) installed within a layer of stone conforming to Sentence (5). (2) Prior to backfilling, the stone layer required by Clause (1)(c) shall be protected in such a manner so as to prevent soil or leaching bed fill from entering the stone by completely covering it with (a) untreated building paper, or (b) a permeable geo-textile fabric. (3) Every pressurized distribution pipe shall be self-draining so as to prevent freezing of its contents. (4) Every pressurized distribution pipe shall, (a) be not less than 1 in. trade size, and (b) have orifices of at least 3 mm in diameter, spaced equally along the length of the pipe to provide even distribution of the effluent. (5) The stone layer required by Clause (1)(c) shall (a) be comprised of washed septic stone, free of fine material, with gradation conforming to Table 8.7.3.4., (b) be not less than 500 mm in width, (c) extend not less than 150 mm below the distribution pipe, and (d) extend not less than 50 mm above the distribution pipe. (6) This Article does not apply to a distribution pipe within a leaching chamber. Table 8.7.3.4. Gradation of Septic Stone Forming Part of Sentences 8.7.3.4.(5) and 8.7.8.2.(6) Particle Size Percent Passing 53 mm 100 19 mm 0-5 75 µm 0-1

(1) Prior to backfilling, the leaching chamber shall be (a) installed level over the length of the absorption trench, (b) securely connected together, section to section, (c) free of structural damage, uncut and used full length, (d) equipped with end caps installed on both ends, (e) equipped with an integrated splash plate at the inlet end of each line of leaching chamber, to prevent soil scouring, and (f) protected in such a manner so as to prevent soil or leaching bed fill from entering the leaching chamber. (2) Except for a shallow buried trench, the distribution pipe within a leaching chamber shall be not less than 3 in. trade size for dosed systems. (3) Every pressurized distribution pipe within a leaching chamber shall (a) extend over the entire length of the leaching chamber, (b) be not less than 1½ in. trade size, (c) have orifices of at least 6 mm in diameter, spaced equally along the length of the pipe to provide even distribution of the effluent, (d) be supported, (e) be self-draining so as to prevent freezing of its contents, and (f) have cleanouts installed at the downstream end of each line of leaching chamber to allow for servicing of the system.

(1) The area described in Sentence 8.7.4.2.(1) shall be designed such that the loading rate does not exceed, for soil having a percolation time set out in Column 1 of Table 8.7.4.1., the maximum value set out opposite it in Column 2 of Table 8.7.4.1. Table 8.7.4.1. Loading Rates for Fill Based Absorption Trenches and Filter Beds Forming Part of Sentences 8.7.4.1.(1) and 8.7.5.2.(2) Percolation Time (T) of Soil, min Loading Rates, (L/m2)/day 1 < T ≤ 20 10 20 < T ≤ 35 8 35 < T ≤ 50 6 T > 50 4 Column 1 2

(1) Except for a shallow buried trench, a leaching bed comprised of absorption trenches may be constructed in leaching bed fill, if unsaturated soil or leaching bed fill complying with Subclause 8.7.2.1.(1)(b)(ii) extends (a) to a depth of at least 250 mm over the area covered by the leaching bed fill, and (b) for at least 15 m beyond the centre lines of the outer distribution pipes or leaching chambers in any direction in which the effluent entering the soil or leaching bed fill will move horizontally. (2) If the unsaturated soil or leaching bed fill described in Sentence (1) has a percolation time greater than 15 min, any additional leaching bed fill added to it to form the leaching bed shall have a percolation time not less than 75% of the percolation time of the unsaturated soil or leaching bed fill to which it is added. (3) Leaching bed fill that does not meet the requirements of Sentence (2) may be used to form the leaching bed if (a) the distance from the bottom of the absorption trench to the underlying soil is not less than 900 mm, or (b) where the distance from the bottom of the absorption trench to the underlying soil is less than 900 mm, the percolation time of the least permeable soil or leaching bed fill within 900 mm from the bottom of the absorption trench is used to calculate the length of the distribution pipe under Article 8.7.3.1. or the length of the leaching chamber under Article 8.7.3.2. (4) Sentence (2) does not apply to any leaching bed fill added as backfill above the stone layer in which the distribution pipe is located. (5) All leaching bed fill added shall be stabilized against erosion. (6) The site to which the leaching bed fill is added shall be generally clear of vegetation. (7) The leaching bed fill that is added shall be compacted in layers in such a manner as to avoid uneven settlement of the distribution pipes or leaching chambers. (8) Any distribution boxes, header lines, absorption trenches, distribution pipes or leaching chambers shall be installed only after the leaching bed fill has been compacted in accordance with Sentence (7). (9) Except as provided in Sentence (10), the sides of the added leaching bed fill shall be sloped to ensure stability, but shall not be steeper than one unit vertically to four units horizontally. (10) The side slope of the leaching bed fill may be increased up to one unit vertically to three units horizontally if measures are taken to prevent erosion and ensure stability of the leaching bed fill. (11) The distances set out in Column 2 of Table 8.2.1.6.B. shall be increased by twice the height that the leaching bed is raised above the original grade.

(1) The total daily design sanitary sewage flow shall not exceed (a) 5 000 L where the treatment unit is a septic tank, or (b) 10 000 L where the treatment unit is a Level II, Level III or Level IV treatment unit as described in Table 8.6.2.2.

(1) The effective area of the surface of the filter medium in each filter bed shall be at least 10 m 2 and not more than 50 m2. (2) The area described in Sentence 8.7.4.2.(1) shall be designed such that the loading rate does not exceed, for soil having a percolation time set out in Column 1 of Table 8.7.4.1., the maximum value set out opposite it in Column 2 of Table 8.7.4.1. (3) Except as provided in Sentence (5), where the total daily design sanitary sewage flow does not exceed 3 000 L, the effective area shall be such that the loading on the surface of the filter medium does not exceed 75 L/m 2 per day. (4) Except as provided in Sentence (5), where the total daily design sanitary sewage flow exceeds 3 000 L (a) the effective area shall be such that the loading on the surface of the filter medium does not exceed 50 L/m 2 per day, and (b) the leaching bed shall be comprised of more than one filter bed, each of similar size and adjacent to each other. (5) Where a Level II, Level III or Level IV treatment unit as described in Table 8.6.2.2. is used in conjunction with a filter bed, the effective area shall be such that the loading on the surface of the filter medium does not exceed 100 L/m2 per day.

(1) Sentences 8.7.4.2.(1), (2) and (4) to (11) apply to the construction of a filter bed. (2) The lines of distribution pipes or leaching chambers shall be evenly spaced over the surface of the filter medium to which the sanitary sewage is applied, with the outer most distribution pipe or leaching chamber not more than 600 mm from the perimeter of that area with a maximum spacing between the centre lines of the distribution pipes or leaching chambers in accordance with Table 8.7.5.3. (3) The filter medium shall have a minimum depth of 750 mm below the stone layer or bottom of the leaching chambers and shall be clean sand comprised of particles ranging in size between the limits of, (a) an effective size of 0.25 mm with a uniformity coefficient not less than 3.5, (b) an effective size of 2.5 mm with a uniformity coefficient not greater than 1.5, and (c) having a uniformity coefficient not greater than 4.5. Table 8.7.5.3. Maximum Spacing Between Centre Lines of Distribution Pipes or Leaching Chambers Forming Part of Sentence 8.7.5.3.(2) Distribution Method Maximum Spacing Between Centre Lines, mm Distribution pipes 1 200 Type I leaching chambers 900 Type II leaching chambers 1 000 (4) The filter medium shall be unsaturated for its entire depth. (5) Where there is more than one filter bed in a leaching bed, the filter beds shall be separated by at least 5 m between the distribution pipes or leaching chambers of the filter beds. (6) The base of the filter medium shall extend to a thickness of at least 250 mm over an area meeting the requirements of the following formula: A = QT/850 where, A = the area of contact in square metres between the base of the filter medium and the underlying soil, Q = the total daily design sanitary sewage flow in litres, and T = the lesser of 50 and the percolation time of the underlying soil. (See Note A-8.7.5.3.(6) and (7)) (7) The stone layer or bottom of the leaching chambers shall be not less than 900 mm above the high ground water table, rock or soil with a percolation time of more than 50 min. (See Note A-8.7.5.3.(6) and (7))

(1) The treatment unit used in conjunction with a leaching bed constructed as a shallow buried trench shall provide an effluent quality that does not exceed the maximum concentrations set out opposite a Level IV treatment unit in Columns 2 and 3 of Table 8.6.2.2. (2) The effluent shall be distributed through a pressurized distribution system having a pressure head of not less than 600 mm when measured to the most distant point from the pump. (3) The pump chamber shall be sized to provide sufficient storage volume so that the effluent is evenly dosed on an hourly basis over a 24-hour period. (4) A shallow buried trench shall not be constructed unless the soil or leaching bed fill is sufficiently dry to resist compaction and smearing during excavation. (5) Every chamber or leaching chamber shall be as wide as the shallow buried trench in which it is contained, and the cross-sectional height of the chamber or leaching chamber at its centre point shall not be less than half the width of the trench. (6) Every chamber or leaching chamber shall contain only one pressurized distribution pipe.

(1) The treatment unit used in conjunction with a leaching bed constructed as a Type A dispersal bed shall provide an effluent quality that does not exceed the maximum concentrations set out opposite a Level IV treatment unit in Columns 2 and 3 of Table 8.6.2.2. (2) A Type A dispersal bed shall be backfilled with leaching bed fill so as to ensure that, after the leaching bed fill settles, the surface of the leaching bed will not form any depressions. (3) The combined thickness of the sand layer and the stone layer if utilized of a Type A dispersal bed shall not be less than 500 mm. (4) Except as provided in Sentence (5), the sand layer shall (a) be comprised of sand that has (i) a percolation time of at least 6 and not more than 10 min, and (ii) not more than 5% fines passing through a 0.074 mm (No. 200) sieve, (b) have a minimum thickness of 300 mm, and (c) have an area that is not less than the lesser of (i) the area of the stone layer determined in accordance with Sentence (6) or, if leaching chambers are used, the area over which the leaching chambers are spaced determined in accordance with Sentence (7), and (ii) the value determined by the formula, A = QT/850 where, A = the area of contact in square metres between the base of the sand and the underlying soil, Q = the total daily design sanitary sewage flow in litres, and T = the lesser of 50 and the percolation time of the underlying soil. (5) Where the underlying soil has a percolation time of more than 15 min, the sand layer referred to in Sentence (4) shall be extended using unsaturated soil or leaching bed fill having a percolation time of not more than 15 min and a depth of at least 300 mm (a) to at least 15 m beyond the perimeter of the treatment unit, or the centre lines of the outer distribution pipes or leaching chambers if utilized, in any direction in which the effluent entering the soil or leaching bed fill will move horizontally, and (b) over an area that is not less than the value determined by the formula, A = QT/400 where, A = the area of contact in square metres between the combined sand layer and extension and the underlying soil, or leaching bed fill if utilized, Q = the total daily design sanitary sewage flow in litres, and T = the lesser of 50 and the percolation time of the underlying soil. (See Note A-8.7.7.1.(5)) (6) Where a stone layer is used, the stone layer shall (a) be rectangular in shape with the long dimension parallel to the site contours, (b) have a minimum thickness of 200 mm, (c) be protected in the manner described in Sentence 8.7.3.4.(2), and (d) be constructed such that the bottom of the stone layer is at least 600 mm above the high ground water table, rock or soil with a percolation time of 1 min or less or greater than 50 min. (e) have a minimum area not less than the value determined by the formula, A = Q/B where, A = the area of the stone layer in square metres, B = the following amount, (i) 50, if the total daily design sanitary sewage flow exceeds 3 000 litres, or (ii) 75, if the total daily design sanitary sewage flow does not exceed 3 000 litres, and Q = the total daily design sanitary sewage flow in litres. (7) Where leaching chambers are used (a) the Type A dispersal bed shall be rectangular in shape with the long dimension parallel to the site contours, and (b) the leaching chambers shall (i) be evenly spaced over the area calculated in Subclause (iv), with a maximum distance of 200 mm between the exterior edges of the lines of leaching chamber, (ii) be protected in the manner described in Clause 8.7.3.5.(1)(f), (iii) be constructed such that the bottom of the leaching chambers is at least 600 mm above the high ground water table, rock or soil with a percolation time of 1 min or less or greater than 50 min, and (iv) have a minimum area not less than the value determined by the formula, A = Q/B where, A = the area over which the leaching chambers are spaced, in square metres, B = the following amount, (i) 50, if the total daily design sanitary sewage flow exceeds 3 000 litres, or (ii) 75, if the total daily design sanitary sewage flow does not exceed 3 000 litres, and Q = the total daily design sanitary sewage flow in litres. (8) Leaching bed fill with a percolation time not exceeding 15 min may be used to satisfy the vertical separation requirements of Clause (6)(d) or Subclause (7)(b)(iii), provided that the leaching bed fill conforms to the requirements specified in Sentence (5) regardless of the percolation time of the underlying soil. (9) Where a stone layer is used, the effluent shall be evenly distributed within the stone layer to within 600 mm of the perimeter of the stone layer. (See Note A-8.7.7.1.(9)) (10) Where leaching chambers are used, the effluent shall be evenly distributed within the area over which the leaching chambers are spaced to within 600 mm of the perimeter of that area. (11) The stone layer or area over which the leaching chambers are spaced shall not be located closer than the minimum horizontal distances set out in Table 8.2.1.6.B. and these distances shall be increased when required by Sentence 8.7.4.2.(11).

(1) Except as provided in Sentence (2) and Sentence 8.7.8.2.(2), a Type B dispersal bed shall conform to the requirements of Article 8.7.2.1. (2) A Type B dispersal bed shall not be located in an area that has an average slope that exceeds one unit vertically to seven units horizontally.

(1) The treatment unit used in conjunction with a leaching bed constructed as a Type B dispersal bed shall provide an effluent quality that does not exceed the maximum concentrations set out opposite a Level IV treatment unit in Columns 2 and 3 of Table 8.6.2.2. (2) A Type B dispersal bed shall be (a) rectangular in shape with the long dimension parallel to the site contours, (b) not more than 1 000 mm in depth measured from the bottom of the stone layer to the finished grade when installed in soil with a percolation time that exceeds 15 min, and (c) backfilled with leaching bed fill so as to ensure that, after the leaching bed fill settles, the surface of the leaching bed will not form any depressions. (3) The bottom of the stone layer shall be at least 600 mm above the high ground water table, rock or soil with a percolation time greater than 50 min. (4) The effluent shall be distributed over the Type B dispersal bed through a pressurized distribution system having a pressure head of not less than 600 mm when measured to the most distant point from the pump. (5) The distribution pipes shall (a) be self-draining so as to prevent freezing of their contents, and (b) have orifices of at least 3 mm in diameter, spaced equally along the length of the pipes. (6) The stone layer containing the distribution pipes shall (a) be comprised of washed septic stone, free of fine material, with gradation conforming to Table 8.7.3.4., (b) extend not less than 250 mm below the distribution pipe, and (c) extend not less than 50 mm above the distribution pipe. (7) The distribution pipes shall be spaced not more than 1.2 m apart with the outermost pipe spaced not more than 600 mm from the edge of the bed. (8) The pump chamber shall be sized to provide sufficient storage volume so that the effluent is evenly dosed on an hourly basis over a 24-hour period. (9) When there is more than one Type B dispersal bed in a leaching bed, the Type B dispersal beds shall be separated by at least 5 m measured from the edge of the stone layers. (10) A Type B dispersal bed shall not be located closer than the minimum horizontal distances set out in Table 8.2.1.6.B. and these distances shall be increased when required by Sentence 8.7.4.2.(11).

(1) The area of a Type B dispersal bed shall not be less than the minimum area determined in accordance with Clause (2)(a) or (b). (2) For the purposes of Sentence (1), the minimum area is either of the following: (a) the area calculated based on the loading rates for Type 2 effluent set out in the Column headed “Type 2” found in Table 2-8 of the BCMOH, “Sewerage System Standard Practice Manual,” or (b) the value determined by the formula, A = QT/400 where, A = the area of contact in square metres between the stone layer and the underlying soil, Q = the total daily design sanitary sewage flow in litres, and T = the percolation time of the underlying soil. (3) The linear loading rates of the underlying soil shall not be greater than (a) the linear loading rates set out in Table 2-11 of BCMOH, “Sewerage System Standard Practice Manual,” where the area of the Type B dispersal bed is determined in accordance with Clause (2)(a), or (b) the following linear loading rate, where the area of the Type B dispersal bed is determined in accordance with Clause (2)(b) (i) 40 L/m, for soil having a percolation time equal to or greater than 24 min, or (ii) 50 L/m, for soil having a percolation time less than 24 min. (4) The width of a Type B dispersal bed shall not exceed 4 m.